Method for preparing a short-chain polysulfide silane mixture

1. Field of the Invention

This invention relates to a method for preparing a mixture of short-chain polysulfide silanes for use in silica-loaded tire compounds.

2. Prior Art

In prior art silica-loaded tire compounds, bis(triethoxysilylpropyl)tetrasulfide is often used as a coupling agent for forming a bond between rubber and silica. This compound, however, has the problem that when milled with rubber and silica at elevated temperatures, it causes the blend to increase its Mooney viscosity to such an extent as to restrain subsequent working.

Then JP-A 259739/1996 discloses a method for improving the workability of a rubber compound loaded with silica and bis(triethoxysilylpropyl)tetrasulfide. When bis(triethoxysilylpropyl)tetrasulfide is used alone, however, workability is improved at the sacrifice of low fuel consumption characteristic of the silica-loaded tires.

We thus paid attention to polysulfide silane mixtures and discovered that a good compromise is made between low fuel consumption and workability when a mixture of polysulfide silanes represented by the following general formula (1) is used.

(RO).sub.3 SiC.sub.3 H.sub.6 S.sub.n C.sub.3 H.sub.6 Si(OR).sub.3(1)

Herein n is a positive number having a distribution whose average is in the range: 2.2≦n≦2.8. and R is methyl or ethyl.

With respect to the preparation of a mixture of polysulfide silanes of formula (1), JP-A 228588/1995 discloses a method involving reacting Na 2 S with sulfur to produce polysulfides and continuously reacting the reaction product with a halogenoalkoxysilane without isolating the polysulfides.

This preparation method, however, has some problems. Because of a substantial variation between lots, the distribution of n in formula (1) sometimes deviates from the desired range. The reaction with the halogenoalkoxysilane is effected in a large amount of solvent, which leads to a low pot yield.

With respect to reaction of sodium polysulfide with halogenoalkoxysilane, JP-A 29726/1973 discloses reaction between Na 2 S 4 or Na 2 S 2 alone and a halogenoalkoxysilane. This patent pertains to the reaction of a single compound, sodium tetrasulfide or sodium disulfide, but teaches nowhere the reaction of polysulfides having a distribution (2.2≦n≦2.8) with a halogenoalkoxysilane.

Therefore, an object of the invention is to provide a novel and improved method for preparing a mixture of short-chain polysulfides of formula (1) in a simple, efficient and steady manner in high yields.

To achieve the above object, we attempted to prepare a mixture of short-chain polysulfide silanes as follows. A metal sulfide is reacted with sulfur in a solvent, for example, to form polysulfides, which are adjusted to a mixture of polysulfides represented by formula (2) or (3):

M.sub.2 S.sub.n (2)

NS.sub.n (3)

wherein M is an alkali metal or ammonium, N is an alkaline earth metal or zinc, and n is a positive number having a distribution whose average falls in the range: 2.2≦n≦2.8. These polysulfides are reacted with a halogeno-propyltrialkoxysilane of the following general formula (4):

(RO).sub.3 SiC.sub.3 H.sub.6 X (4)

wherein X is a halogen atom and R is methyl or ethyl. Then, a mixture of short-chain polysulfide silanes represented by the following general formula (1):

(RO).sub.3 SiC.sub.3 H.sub.6 S.sub.n C.sub.3 H.sub.6 Si(OR).sub.3(1)

wherein n and R are as defined above is produced in an efficient and steady manner in high yields. Because of its distribution, the short-chain polysulfide silane mixture is effective for achieving both low fuel consumption and workability when blended in silica-loaded rubber compounds.

Accordingly, the present invention provides a method for preparing a mixture of short-chain polysulfide silanes of formula (1), comprising the step of reacting polysulfides of formula (2) or (3) with a halogenopropyl-trialkoxysilane of formula (4).

Since the polysulfide silane mixture obtained by the inventive method is not a single compound (wherein n is an integer), inexpensive manufacture is possible. The invention is advantageous when it is desired to control the distribution of a short-chain polysulfide silane mixture. When a mixture of polysulfides of formula (2) or (3) is isolated and adjusted, the distribution of this polysulfide mixture can be controlled. Then the distribution of a short-chain polysulfide silane mixture can be easily controlled, as compared with the case wherein the polysulfide mixture is not isolated. In the prior art where the polysulfide mixture is not isolated, a large amount of solvent such as alcohol must be used in order to dissolve sulfides such as sodium sulfide (Na 2 S) in the system. Since polysulfides such as sodium polysulfides wherein n is in the range of 2.2 to 2.8 are more dispersible in the solvent than the sulfides such as sodium sulfide (Na 2 S), the amount of the solvent used for reaction with halogenopropyltrialkoxysilane can be reduced, leading to an improved pot yield.

It might occur to those skilled in the art to produce a mixture of short-chain polysulfide silanes by desulfurizing tetrasulfide silane with sodium cyanide. As compared with the desulfurizing method, the method of the invention ensures low cost and safety in preparing a mixture of short-chain polysulfide silanes.

In the method for preparing a mixture of short-chain polysulfide silanes according to the invention, a mixture of metal polysulfides of formula (2) or (3) is first isolated and adjusted so that free sulfur is absent and the average of distribution of n may fall in the range of 2.2 to 2.8. The metal polysulfide mixture is then reacted with a halogenopropyltrialkoxysilane of formula (4) in a solventless system or in a solvent, thereby producing a mixture of short-chain polysulfide silanes of formula (1).

(RO).sub.3 SiC.sub.3 H.sub.6 S.sub.n C.sub.3 H.sub.6 Si(OR).sub.3(1)

M.sub.2 S.sub.n (2)

NS.sub.n (3)

(RO).sub.3 SiC.sub.3 H.sub.6 X (4)

In formulas (1) to (4), n is a positive number having a distribution whose average falls in the range: 2.2≦n≦2.8, R is methyl or ethyl, M is an alkali metal or ammonium, N is an alkaline earth metal or zinc, and X is a halogen atom.

Examples of the metal polysulfides represented by formulas (2) and (3) include Na 2 S n , K 2 S n , Li 2 S n , (NH 4 ) 2 S n , CaS n , MgS n , and ZnS n , with Na 2 S n being especially preferred.

The manner of preparing metal polysulfides is not critical. In one example, they can be prepared by reacting a metal sulfide represented by M 2 S or NS wherein M and N are as defined above with sulfur in a solvent. The molar ratio of metal sulfide to sulfur is determined as appropriate. When sodium sulfide is used, for example, the desired mixture of sodium polysulfides can be obtained by reacting 1 mol of sodium sulfide with 1.2 to 1.8 mol of sulfur.

In this reaction, the solvent permitting the metal sulfide to be partially dissolved in the reaction system is advantageously used. Illustrative examples of the solvent include aliphatic solvents such as pentane and hexane, aromatic solvents such as benzene, toluene, and xylene, ethers such as diethyl ether and dibenzyl ether, esters, and ketones. The most advantageous solvents are alcohols such as methanol, ethanol, propanol, butanol, benzyl alcohol, and phenol, with methanol and ethanol being especially advantageous.

The reaction temperature may range from room temperature to 150° C., more preferably from 60 to 100° C. Solvent reflux conditions, especially ethanol reflux conditions are appropriate. Unless the metal sulfide is partially dissolved in the reaction system, the reaction between sulfur and metal sulfide does not proceed to a full extent, leaving some sulfur unreacted and failing to produce a mixture of metal polysulfides having the desired distribution.

At the end of reaction, the solvent used is distilled off in vacuum, leaving the mixture of metal polysulfides. The metal polysulfide mixture thus obtained can be isolated and adjusted in distribution by a sublimation technique utilizing a difference in sublimation temperature.

After the distribution adjustment, the metal polysulfide mixture is reacted with a halogenopropyl-trialkoxysilane of formula (4):

(RO).sub.3 SiC.sub.3 H.sub.6 X (4)

wherein X is a halogen atom such as Cl, Br or I, preferably Cl or Br, and R is methyl or ethyl. Examples of the compound of formula (4) include ClC 3 H 6 Si(OC 2 H 5 ) 3 , ClC 3 H 6 Si(OCH 3 ) 3 , and BrC 3 H 6 Si(OC 2 H 5 ,) 3 .

When the metal polysulfide mixture is reacted with the halogenopropyltrialkoxysilane, a solvent may or may not be used. When used, the solvent permitting the metal sulfide to be partially dissolved in the reaction system is advantageous. Examples of the solvent are as described above. Illustrative examples of the solvent include aliphatic solvents such as pentane and hexane, aromatic solvents such as benzene, toluene, and xylene, ethers such as diethyl ether and dibenzyl ether, esters, and ketones. The most advantageous solvents are alcohols such as methanol, ethanol, propanol, butanol, benzyl alcohol, and phenol, with methanol and ethanol being especially advantageous.

The reaction temperature may range from room temperature to 150° C., more preferably from 60 to 100° C. Solvent reflux conditions, especially ethanol reflux conditions are appropriate. The reaction time is usually 1 to 20 hours although reaction may proceed to completion within 1 to 5 hours under ethanol reflux conditions.

After the completion of reaction, the solvent is distilled off under vacuum and the salt formed is filtered off, obtaining the desired mixture of short-chain polysulfide silanes of formula (1) in high yields.

With respect to the short-chain polysulfide silane mixture of formula (1), if n is less than 2.2, tires having the polysulfide silane mixture blended therein are unsatisfactory in fuel consumption reduction. If n is more than 2.8, the rubber milled together with the polysulfide silane mixture at elevated temperature becomes poorly workable. The more preferred range of n is from 2.3 to 2.7, within which the best compromise between low fuel consumption and workability is found.

There has been described a method for preparing a mixture of short-chain polysulfides of formula (1) in a simple and steady manner in high yields.

Examples of the invention are given below by way of illustration and not by way of limitation.